Conventionally, a spark plug is used for providing ignition in an internal combustion engine such as a gasoline engine. In a spark plug, a spark discharge gap is formed between a center electrode and a ground electrode. Incidentally, there is known a spark plug in which a noble metal tip is attached on the electrode base metal of a ground electrode via an intermediate member (refer to, for example, International Publication WO02009/084565). The intermediate member is used for reducing the occurrence of a defect which could otherwise result from the noble metal tip being attached directly onto the electrode base metal. For example, through use of the intermediate member therebetween, while the amount of use of a noble metal is reduced, the area of joining to the electrode base metal can be increased. According to the technique mentioned in International Publication WO02009/084565, the intermediate member to which the noble metal tip is attached is joined to the electrode base metal by welding.
However, as a result of recent tendency toward higher engine outputs, the working environment of a spark plug is becoming more severe than before. Accordingly, the spark plug manufactured on the basis of the technique disclosed in International Publication WO02009/084565 has potentially suffered separation of the intermediate member from the electrode base metal as a result of the welding strength between the intermediate member and the electrode base metal failing to endure a working environment. Such a problem is not limited to the ground electrode, but is in common with a spark plug in which the noble metal tip is attached on the electrode base metal of the center electrode via the intermediate member.
Therefore, an advantage of the present invention is a technique for improving welding strength between the intermediate member and the electrode base metal.